der Hörer
receiver
The part of a traditional telephone that you pick up, hold to your ear, and speak into.
Als das Telefon klingelte, nahm sie sofort den Hörer ab.
When the phone rang, she immediately picked up the receiver.

receiver / listener
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receiver
The part of a traditional telephone that you pick up, hold to your ear, and speak into.
Als das Telefon klingelte, nahm sie sofort den Hörer ab.
When the phone rang, she immediately picked up the receiver.

The word 'Hörer' derives directly from the verb 'hören' (to hear/listen). The suffix '-er' forms a noun here that refers either to the person who listens or to the device used for hearing.
Wer war der erste Hörer, der die richtige Antwort wusste?
Who was the first listener who knew the correct answer?
The context makes it clear which meaning is intended. Words like 'Telefon' (telephone) or 'auflegen' (to hang up) point to the telephone receiver. 'Radio' or 'Podcast' point to the listener, and 'Universität' (university) or 'Vorlesung' (lecture) point to the auditor.
Er legte wütend den Hörer auf.
He angrily hung up the receiver.
In connection with the telephone, the most important verbs are 'den Hörer abnehmen' (to pick up the receiver; to start a call) and 'den Hörer auflegen' (to hang up the receiver; to end a call).
Als es klingelte, hat niemand den Hörer abgenommen.
When it rang, no one picked up the receiver.
In radio shows or podcasts, the direct address 'Liebe Hörerinnen und Hörer' (dear listeners) is a very common and classic greeting.
„Liebe Hörerinnen und Hörer, willkommen zu unserer heutigen Sendung!“
'Dear listeners, welcome to today's program!'
telephone
'das Telefon' and 'der Hörer' are both connected to a traditional phone call. 'das Telefon' means the whole device. 'der Hörer' is only the part of an older telephone that you pick up, hold to your ear, and speak into. The words can be confusing because you make phone calls with both, but they refer to different things. With modern smartphones, you normally do not say 'Hörer'. 'Hörer' is mainly used for landline phones or older telephones with a removable part.

Nach dem Gespräch legte Herr Schneider den Hörer zurück auf das alte Telefon.
After the conversation, Mr. Schneider put the receiver back on the old telephone.
Vor dem Termin legt sie das Telefon leise auf den Schreibtisch.
Before the appointment, she quietly places the telephone on the desk.